The present invention relates generally to systems and methods for vapor depositing thin films, and more particularly to an improved source cell for thin film deposition by molecular beam epitaxy.
Molecular beam epitaxy (MBE) is a physical vapor deposition technique performed in ultra high vacuum wherein a molecular beam flux of one or more constituent elements or compounds of interest from a heated source (e.g. effusion cell, Knudsen cell, source cell) is directed onto a heated (usually single crystal) substrate on which the atoms or molecules comprising the flux condense partially or completely and chemically react to form a deposited film. Film quality depends on several variables including source material, substrate temperature, beam flux ratio and growth rate, which often need to be determined experimentally. Various materials may be deposited as thin films using MBE, including oxides, semiconductors, insulators and metals. MBE technique allows precise control of film composition, crystallinity and crystal perfection, and production of films of extreme purity with morphologies at open and buffed interfaces that are smooth to within one atomic layer.
In the practice of an MBE process, the molecular beam flux from the heated source must be accurately controlled. In typical prior art processes, the flux is controlled by monitoring source temperature using a refractory metal (e.g. tungsten and rhenium alloy) thermocouple contacting the crucible containing the source material, which control suffers from numerous disadvantages and limitations. First, the crucible typically comprises an insulating material, such as pyrolitic boron nitride, and the thermocouple is held in contact with the crucible, which arrangement, combined with poor thermal transfer properties of the crucible material, results in inconsistent, variable and unreliable temperature readings for the source material. Second, the refractory metal thermocouple normally generates a very low output voltage, but must be used because only low vapor pressure, high temperature, non-contaminating materials may be used in contact with the heated MBE source. Electrical or electromagnetic noise in the MBE system may therefore interfere with the accuracy of the thermocouple readings. Third, the thermocouple is subject to brittle failure resulting in part from condensation of metallic and nonmetallic vapors on and alloying with the thermocouple. Fourth, crucible failure may result in hot, reactive liquid metals contacting the thermocouple, with consequent failure of the thermocouple and source cell.
The invention solves or substantially reduces in critical importance problems with prior art MBE processes and systems by providing an optical sensor for monitoring source temperature. The sensor provides greatly improved (10 to 50 times) sensitivity, reproducibility, stability and precision in temperature control of MBE effusion cells, increased reliability and ruggedness and improved resistance to electronic and electromagnetic interference, and is easily cleaned if contaminated. The invention provides new levels of precision and control of film composition and thickness and allows control of stoichiometry for certain non-lattice matched film growths. The high sensitivity and fast response time of the sensor substantially reduces flux transient effects in MBE source cells.
It is therefore a principal object of the invention to provide an improved source cell for vapor depositing thin films by MBE.
It is a further object of the invention to provide an optical sensor for monitoring source temperature in an MBE system.
It is a further object of the invention to provide improved control of the flux from a molecular beam epitaxy effusion cell.
It is yet another object of the invention to provide an MBE system having substantially reduced flux transients associated with the MBE source.
It is another object of the invention to provide improved temperature control in an MBE source cell.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.